Master student Projects

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Projects for Master students in the Nikhef B-physics (LHCb) group 2020 - 2021

date: April 2020

This is an overview with all available Master student projects in the Nikhef B-physics (LHCb) group.

If you have your own research proposal, need more detailed information on the (availability) of individual proposals or would like to discuss about other available projects in the group you are always welcome to contact either the contact person for the project and/or the Nikhef B-physics group leader:

Marcel Merk [e-mail:, Tel 020-5925107, Nikhef room N243]

For an overview of the theses written in the Nikhef B-physics group you can look at the Nikhef LHCb theses page

(The full list of projects at Nikhef can be found here.)

Master projects in the Nikhef B-physics group

1) Measurement of delta md

Supervisor : Niels Tuning (staff), Emmy Gabriel (postdoc), Michele Veronesi (PhD)

Research Description:

The decay B0->D-pi+ is very abundant in LHCb, and therefore ideal to study the oscillation frequency delta md, with which B0 mesons oscillate into anti-B0 mesons, and vice versa. This process proceeds through a so-called box diagram which might hide new yet-undiscovered particles. Recently, it has been realized that value of delta md is in tension with the valu of CKM-angle gamma, triggering renewed interest in this measurement.

Our group at Nikhef has 10 years experience in the analysis of B->Dh decays. This analysis may lead to a publication on behalf of the LHCb collaboration.

Relevant information:

[1] arXiv:1812.06963 M. Blanke, A. Buras, Emerging delta md Anomaly from Tree-Level Determinations of |Vcb| and the angle gamma

[2] arXiv:1911.07856 D. King, A. Lenz, Th. Rauh, |Vcb| and gamma from mixing

[3] talk N. Tuning, CKM unitarity and B mixing

[4] arXiv:1210.6750 LHCb coll., Measurement of the B0--B¯0 oscillation frequency delta md with the decays B0->D-pi+ and B0->J/psiK*0

[5] arXiv:1805.03448 LHCb coll., Measurement of CP violation in B0->D±pi± decays

2) Searching for CPT violation

Supervisor : Niels Tuning (staff), Emmy Gabriel (postdoc), Michele Veronesi (PhD)

Research Description:

CPT symmetry is closely linked to Lorentz symmetry, and any violation would revolutionize science. There are possibilities though that supergravity could cause CPT violating effects in the system of neutral mesons. The precise study of B0s oscillations in the abundant Bs->Dspi decays can give the most stringent limits on Im(z) to date.

Our group at Nikhef has 10 years experience in the analysis of B->Dh decays. This analysis may lead to a publication on behalf of the LHCb collaboration.

Relevant information:

[1] arXiv:1603.04804 LHCb coll., Search for violations of Lorentz invariance and CPT symmetry in B0(s) mixing

[2] arXiv:1407.1269 J. van Tilburg and M. van Veghel, Status and prospects for CPT and Lorentz invariance violation searches in neutral meson mixing

3) BR(B0->D-pi+) and fd/fu with B+->D0pi+

Supervisor : Niels Tuning (staff), Emmy Gabriel (postdoc), Michele Veronesi (PhD)

Research Description:

The abundant decay B0->D-pi+ is often used as normalization channel, given its clean signal, and well-known branching fraction, as measured by the B-factories. However, this branching fraction can be determined more precisely, when comparing to the decay B+->D0pi+ , which has a twice better precision.

In addition, the production of B0 and B+ mesons is often assumed to be equal, based on isospin symmetry. The study of B+->D0pi+ and B0->D-pi+ allows for the first measurement of this ratio, fd/fu.

Our group at Nikhef has 10 years experience in the analysis of B->Dh decays. This analysis may lead to a publication on behalf of the LHCb collaboration.

Relevant information:

[1] CERN-THESIS-2018-137 J. Butter, Branching fraction measurement of B0->D+s pi?

5) Searches for new pentaquarks

Supervisor : P. Koppenburg (staff)

Research Description:

In 2015 LHCb surprisingly discovered states containing five quarks, called Pc+ pentaquarks. Such particles question our understanding of confinement, the principle that forces quarks to remain in a single hadron. Which hadrons are allowed and which are not? The pentaquarks were found in the decay of the Lambda_b baryon to a Pc+ and a kaon, and Pc+ to a J/psi and a proton. This project aims at studying other similar but yet unobserved decays which could reveal the presence of the know Pc+, or yet unknown pentaquarks. The student will optimise a selection for finding such a decay in LHCb data using machine learning techniques.

Relevant information:

[1] arXiv:1507.03414 (the pentaquark paper)

[2] arXiv:1406.0755 (a paper by P. Koppenburg similar to what the project could lead to).

6) Measurement of the relative efficiency of muons and electrons with J/psi->ll decays

Supervisors: Patrick Koppenburg

Research description:

LHCb has produced intriguing results hinting at violation of lepton universality. As extraordinary claims require extraordinary evidence we propose to perform a measurement of the relative reconstruction efficiency of electrons and muons by comparing the number of J/psi particles counted in the di-electron and di-muon final states.

7) Very rare decays of B mesons

Supervisors: Marcel Merk

Research description: Decays of B-mesons to a pair of muons or a pair of electrons are very strongly suppressed in the Standard Model. As a consequence these decays are sensitive to virtual contributions of particles beyond the Standard Model. The project includes a theoretical study on these sensitivties and/or experimental improvements to the measurement.

8) Measurement of Central Exclusive Production Rates of Chi_c using converted photons in LHCb.

Supervisor: Kazu Akiba

Research description: Central exclusive production (CEP) of particles at the LHC is characterised by a extremely clean signature. Differently from the typical inelastic collisions where many particles are created resulting in a so-called Primary Vertex, CEP events have only the final state particles of interest. In this project the particle of interest is a pair of charmed quarks creating a chi_c particle. In theory this process is generated by a long range gluon exchange and can elucidate the nature of the strong force, described by the quantum chromodynamics in the the standard model. The proposed work involves analysing a pre-existing dataset with reconstructed chi_c and simulating events at the LHCb in order to obtain the relative occurrence rate of each chi_c species (spins 0, 1, 2), a quantity that can be easily compared to theoretical predictions.

9) Optimization studies for Vertex detector at the High Lumi LHCb

Supervisor: Kazu Akiba

Research description: The LHCb experiment is dedicated to measure tiny differences between matter and antimatter through the precise study of rare processes involving b or c quarks. The LHCb detector will undergo a major modification in order to dramatically increase the luminosity and be able to measure indirect effects of physics beyond the standard model. In this environment, over 42 simultaneous collisions are expected to happen at a time interval of 200 ps where the two proton bunches overlap. The particles of interest have a relatively long lifetime and therefore the best way to distinguish them from the background collisions is through the precise reconstruction of displaced vertices and pointing directions. The new detector considers using extremely recent or even future technologies to measure space (with resolutions below 10 um) and time (100 ps or better) to efficiently reconstruct the events of interest for physics. The project involves changing completely the LHCb Vertex Locator (VELO) design in simulation and determine what can be the best performance for the upgraded detector, considering different spatial and temporal resolutions.

10) Measurement of charge multiplication in heavily irradiated sensors

Supervisor: Kazu Akiba

Research description: During the R&D phase for the LHCb VELO Upgrade detector a few sensor prototypes were irradiated to the extreme fluence expected to be achieved during the detector lifetime. These samples were tested using high energy particles at the SPS facility at CERN with their trajectories reconstructed by the Timepix3 telescope. A preliminary analysis revealed that at the highest irradiation levels the amount of signal observed is higher than expected, and even larger than the signal obtained at lower doses. At the Device Under Test (DUT) position inside the telescope, the spatial resolution attained by this system is below 2 um. This means that a detailed analysis can be performed in order to study where and how this signal amplification happens within the 55x55 um^2 pixel cell. This project involves analysing the telescope and DUT data to investigate the charge multiplication mechanism at the microscopic level.

11) Searching for lepton-universality violation with ?b ? ?l+l? decays

Supervisor : Niels Tuning (staff), Lex Greeven (PhD), Mick Mulder (postdoc@CERN)

Research Description: A current “hot topic” in the field of particle physics is the potential violation of lepton-universality. At the LHCb experiment, lepton-universality tests are performed by looking at the ratio of decays into muons and into electrons/taus. Recent measurements in meson modes show hints (2 ? 3?) of lepton non-universality. Baryonic modes, however, have been less studied and provide an independent test of lepton-universality. At Nikhef, we study the decay ?b ? ?l+l- , where l can be an electron or a muon. There are two possible project topics:

1. Identifying novel analysis techniques in the high di-lepton invariant mass region. Electrons in this region undergo more Bremsstrahlung, and therefore have a worse momentum resolution, meaning background from the resonant ? (2s) mode can leak into our signal. Since we expect most of our signal in this region, it is important to improve this, most likely using machine learning techniques.

2. Identifying, simulating, and setting up a rejection for partially reconstructed ?0b ? ?*l+l? backgrounds. By not fully reconstructing the excited ?0, we can mis-reconstruct it as a signal candidate. Machine learning techniques could be explored.

Relevant information:

[1] arXiv:1903.09252 LHCb coll., Search for lepton-universality violation in B+?K+l+l?decays

[2] arXiv:1903.09252 LHCb coll., Test of lepton universality with B0?K*0l+l? decays

[3] arXiv:1912.08139 LHCb coll., Test of lepton universality with ?b?pKl+l? decays

[4] arXiv:1602.01399 W. Detmold and S. Meinel, ?b??l+l? form factors, differential branching fraction, and angular observables from lattice QCD with relativistic b-quarks

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